1. Field of the Invention
The present invention relates to a block distortion corrector and an image signal expander, and more particularly to a block distortion correction processor which corrects a distortion between image blocks in an image signal expander which provides expanding processing for each image block, the correction processing being done for compressed image data which are highly compressed and encoded for each image block such that image information can be stored or transmitted as digital data.
2. Description of the Related Art
Due to a huge amount of data when digital image data are transmitted or stored, a variety of methods such as an image data encoding method which utilizes 2-dimensional orthogonal transformation formula have conventionally been devised as a method of compressing the digital image data. Among them, an encoding method which utilizes DCT (Discrete Cosine Transform) which is also utilized in H.261 (TV telephone/compression of moving pictures for TV conference), which is the International Standards, is well-known.
In a data transmission utilizing such encoding method, an input image data which consists of a plurality of pixels in matrix arrangement and corresponds to one display image is divided into image block units of 8.times.8 pixels on the transmission side of the image data. Then a data transformation processing using DCT, a quantization processing, and a variable-length encoding processing are performed on the input image data for each image block, and the input image data are transmitted. On the reception side of the compressed image data, an expansion processing is performed on the compressed image data for each image block divided as above. In other words, a decoding, an inverse quantization, and an inverse DCT are successively performed on the encoded data for each image block to generate a recovered image data.
When the encoding described above is being performed, if the compression rate is raised by widening the quantization step width for higher efficiency, a deterioration of a reproduced image occurs because of a linear summation of the DCT output during the inverse transformation for the pixel reproduction. Furthermore, since these expansion processes are performed in units of image blocks, there arises a problem that a distortion of the recovered image data occurs between the image blocks, notably degrading the recovered image data.
In order to reduce such block distortion, a method of alleviating the appearing of an edge at the image block boundary can be devised in which the recovered image data are processed by a low-pass filter.
A specific constitution of such method is disclosed in Japanese Laid-Open Patent Publication No. 5-14735. FIG. 7 is a block diagram describing an image expanding section of the image processor which is disclosed in the Laid-Open Publication.
Reference numeral 20 designates the image expansion section of this image processor, which is constituted such that image data which are compression-processed for each image block and transmitted are expansion-processed for each image block. The image expansion section 20 includes a decoder 21 which decodes the encoded data which is the compressed image data from the transmission side for each image block, an inverse quantization device 22 which performs an inverse quantization processing on the decoded data for each image block, and an inverse DCT device 23 which performs an inverse DCT processing on the output thereof.
The image expansion section 20 is further provided with an image buffer 24 which outputs the image data D6 of the recovered image block after having stored a necessary amount for the block distortion correction. A low-pass filter 25 as a block distortion corrector is connected to the output of the image buffer such that the image data D6 are processed for block distortion correction by the low-pass filter 25 and output.
Next, the operation of the device of FIG. 7 will be described.
First, the encoded data D1 are decoded by the decoder 21 for each image block and output to the inverse quantization device 22 as the decoded data D2. The inverse quantization process is performed on the decoded data D2 In the inverse quantization device 22, and the inverse quantization data D3 thereby obtained are output to the inverse DCT device 23. In the inverse DCT device 23, the inverse DCT processing on the inverse quantization data D3 is performed utilizing the inverse quantization data D3 and the matching data D5 from the image buffer 24, and the recovered image block data D4 are output to the image buffer 24. The image buffer 24 stores a necessary amount of the recovered image block data D4 for block distortion correction, and the data D6 for block distortion correction are supplied to the low-pass filter (the block distortion corrector) 25. In the low-pass filter 25, the block distortion correction processing of the recovered image data is performed utilizing the data D6, and post-correction recovery data D7 corrected for the block distortion are output.
However, in a method of correcting a block distortion using the above-described low-pass filter 25, there is a fundamental problem that it cannot be decided whether a discontinuity of pixel data occurring at the boundary of the image blocks processed for expansion is due to a block distortion, or it is due to the characteristics of the original image such as in the case that the luminance greatly changes in the vicinity of the boundary. For example, if a uniform block distortion correction processing is performed on recovery image data for one display screen, the filter works weakly for a part of the image of the display screen and the block distortion is not adequately removed; but the filter works too strong for another part of the image and the image in the vicinity of the block boundary blurs. These problems pose a large obstacle in improving an image quality of a recovered image.
One of the conventional image signal expander performs a block distortion correction processing while judging whether or not the block distortion correction be conducted instead of a uniform block distortion correction processing as described above. Such block distortion correction method will be described with reference to FIGS. 8 and 9.
FIG. 8 illustrates block boundary pixels of the image block subjected to a block distortion correction processing, and FIG. 9 illustrates the arrangement of other image blocks in the vicinity of the image block.
This method includes judging whether or not a block distortion correction is conducted on 28 pixels a1 (marked with "*" in the FIG. 8) which are located on the periphery of the image block A based on the image data of the image blocks adjacent to the image block a when performing the block distortion correction processing on the pixels a1.
Although the decision of whether the discontinuity of the pixel data occurring at the boundary of the image blocks is due to a block distortion or it is due to the characteristics of the original image, as in a case where the luminance greatly changes in the vicinity of the block boundary can possibly be made on the transmission side of the image data based on the image date of the original image. It is not easy on the reception side of the image data to decide whether the discontinuity of the pixel data is due to a block distortion or due to the characteristics of the original image because data representing the original image does not exist.
Consequently, if the block distortion correction is to be performed only when the values of the image data between neighboring pixel blocks are a certain value or more, as described above, in a case where the luminance greatly changes in the vicinity of the block boundary because of the characteristics of the original image, an image quality of a recovered image greatly deteriorates due to the block distortion correction processing.
Moreover, in order to realize a block distortion correction processing as illustrated in FIG. 8, it is necessary that, with respect to the image block A which is about to be in a process for block distortion correction, the image blocks B, D, F, and H in the vicinity thereof be already present as a reproduced image (FIG. 9).
In other words, the compressed image signals which are successively transmitted from the transmission side in units of image blocks are arranged, for example, from the left end to the right end in each row and from the top row to the bottom row in units of image blocks on a display screen illustrated in FIG. 10. There are a variety of other ways of sequencing the transmission of the image signals corresponding to image blocks other than the one illustrated in FIG. 10. For example, the image signals may be arranged from the top end to the bottom end and from the left column to the right column on a display screen, or they may be arranged in the same way as above except that the arrangement in the horizontal direction goes in the opposite direction or that the arrangement in the vertical direction goes in the opposite direction.
As a result, for example, if a block distortion correction on the 15th image block shown in FIG. 10 is to be made, the block distortion correction processing must wait until the compressed image signal of the 26th image block is reproduced by the axpansion processing.
That is, the block distortion correction on the 15th image block is performed after waiting for a time period necessary for image reproduction processing for eleven image blocks from the time of receiving the reproduction signal corresponding to the 15th image block.
As a result, particularly when moving picture information is being handled in data communication, a wait time until the block distortion correction is performed as described above poses a significant problem.